Sensor devices, including accelerometers, based on capacitive pick-off and electrostatic closed-loop rebalance of out of plane pendulous masses that use single or multiple stacked parallel plates for pick-off are generally well known.
FIGS. 1, 2 and 3 illustrate, in accordance with prior art, a capacitive pick-off sensor constructed as a conventional mid-pendulum hinged or “teeter-totter” type accelerometer having the suspending flexure positioned intermediate the acceleration sensing element, wherein FIG. 1 is an exploded dynamic side view of the sensor, FIG. 2 is a static plan view, and FIG. 3 is a static sectional side view taken along a longitudinal axis L of the accelerometer of FIG. 2. Such devices are constructed using microcircuit techniques to produce reliable, maintenance-free capacitive acceleration-sensing devices. Such a capacitive acceleration sensing device 1, hereinafter a capacitive accelerometer, includes a pair of stationary substrates 2, 3 having opposed parallel planar faces. The substrates 2, 3 are spaced from one another and each has a number of metal electrode layers 4, 5 of predetermined configuration deposited on one surface to form respective capacitor electrodes or “plates.” This is an example of multiple stacked plates. The electrode elements 4 (or 5) operate as an excitation electrode to receive stimulating signals, and the other electrode elements 5 (or 4) operate as the feedback electrodes for electrostatic rebalance. A single set of electrode elements 4 (or 5) operates as both excitation and feedback electrodes when the feedback signal is superimposed on the excitation signal.
A pendulous acceleration sensing element 7, commonly referred to as either a “pendulum” or a “proof mass,” which operates as pick-off electrode, is flexibly suspended between the substrates 2, 3 by one or more rotational flexures 6 situated at elevated attachment points 8 for pendulous rotation about a fulcrum or hinge axis h to form different sets of capacitors with electrode elements 4, 5. Movement of the acceleration-sensing element, or “pendulum,” 7 in response to acceleration changes its position relative to the stationary excitation electrodes 4 (or 5), thereby causing a change in pick-off capacitance. This change in pick-off capacitance is indicative of acceleration. A set of capacitors for electrostatic rebalance is made up of the sensing element 7 and the feedback electrodes 5 (or 4) for driving the sensing element 7 to its reference position balanced between the electrode elements 4, 5 and maintaining it there.
In such an acceleration sensor device, a capacitance formed by the excitation electrodes 4 (or 5) and the moveable sensing element 7 is related to 1/D, where D is the offset between electrodes 4, 5 and the pendulous acceleration sensing element 7 when constructed or emplaced relative to the substrates 2, 3 on the elevated attachment points 8.
FIGS. 4, 5 illustrate another capacitive pick-off sensor 15 constructed in accordance with prior art as a conventional cantilevered sensing element 16 suspended by one or more flexures 17 each constructed as a bending beam, wherein FIG. 4 is a static side view, and FIG. 5 is a dynamic side view of the sensor 15 showing the cantilevered sensing element 16 being deflected to an exaggerated degree. Such a cantilevered capacitive acceleration sensing device 15, hereinafter a capacitive accelerometer, includes at least one, and optionally two, stationary substrates 18 having opposed parallel planar faces. The substrates 18 are spaced from one another and the cantilevered sensing element 16, and each has a number of the metal electrode layers 4, 5 of predetermined configuration deposited on one surface to form respective capacitor electrodes or “plates.” As described above, the electrode elements 4 (or 5) operate as an excitation electrode to receive stimulating signals, and the other electrode elements 5 (or 4) operate as the feedback electrodes for electrostatic rebalance. A single set of electrode elements 4 (or 5) operates as both excitation and feedback electrodes when the feedback signal is superimposed on the excitation signal. The cantilevered sensing element 16, which operates as pick-off electrode, is flexibly suspended above one substrate 18, or between both substrates 18, at elevated attachment points 19 for pendulous rotation about its fulcrum or hinge axis h to form different sets of capacitors with electrode elements 4, 5. As in the teeter-totter type acceleration sensor device 1, the fulcrum or hinge axis h of the cantilevered sensing element 16 is assumed to coincide with the centerline of the flexure 17.
As in the sensor 1 of FIGS. 1, 2 and 3, movement of the acceleration-sensing element, or “pendulum,” 16 in response to acceleration changes its position relative to the stationary excitation electrodes 4 (or 5), thereby causing a change in pick-off capacitance. This change in pick-off capacitance is indicative of acceleration. A set of capacitors for electrostatic rebalance is made up of the sensing element 16 and the feedback electrodes 5 (or 4) for driving the sensing element 16 to its reference position balanced between the electrode elements 4, 5 and maintaining it there.
As in the teeter-totter type acceleration sensor device, a capacitance formed by the excitation electrodes 4 (or 5) and the moveable sensing element 16 is related to 1/D, where D is the offset between electrodes 4, 5 and the pendulous acceleration sensing element 16 when constructed or emplaced relative to the one or more substrates 18 on the elevated attachment points 19.
A desirable characteristic of an accelerometer is a linear response for pick-off capacitance C versus acceleration input g. However, conventional high-g range teeter-totter and cantilevered type accelerometers have less than optimum linearity for high performance application and may also have a non-monotonic response for electrostatic rebalance force versus acceleration when feedback voltage is capped. The capacitance seen by the pick-off electrodes is related to the integral of 1/d(i) for each a(i) over the area of the excitation electrodes, where d(i) is the dynamic separation distance between the stationary electrodes and the pendulum for each incremental area a(i). The sensor's dynamic range, scale factor and response linearity are thus defined by the separation distance D (shown in FIG. 1) between the stationary electrode elements 4, 5 and the respective pendulous acceleration-sensing element 7, 16, and the lateral offset of electrode elements 4, 5 relative to the respective attachment points 8, 19. In both conventional teeter-totter and cantilevered type acceleration sensor devices, the stationary capacitor electrodes 4, 5 are traditionally arranged substantially along a longitudinal axis of symmetry L of the respective acceleration sensing device 1, 15 perpendicular to the fulcrum or hinge axis h of flexures 6, 17 suspending the respective acceleration-sensing elements 7, 16, as illustrated in FIGS. 1, 4. Electrode elements 4, 5 are sized and spaced in symmetrically with respect to the longitudinal axis L of the respective acceleration sensing device 1, 15. Furthermore, the electrode elements 4 (or 5) are further sized and spaced symmetrically with respect to the fulcrum or hinge axis h of the respective moveable sensing element 7, 16, which is assumed to lie along a geometric centerline Cg of the respective flexure 6, 17.
Conventional teeter-totter type acceleration sensor devices of the type illustrated in FIG. 1 and cantilevered type acceleration sensor devices of the type illustrated in FIG. 4, have been fabricated from a body of semiconductor material, such as silicon, as Micro Electro-Mechanical Systems, or “MEMS,” integrated micro devices or systems combining electrical and mechanical components fabricated using integrated circuit (IC) batch processing techniques.
In the most general form, MEMS consist of mechanical microstructures, microsensors, microactuators and electronics integrated in the same environment, i.e., on a silicon chip. MEMS is an enabling technology in the field of solid-state transducers, i.e., sensors and actuators. The MEMS microfabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks but in combination can accomplish complicated functions. Current applications include accelerometers, pressure, chemical and flow sensors, micro-optics, optical scanners, and fluid pumps. For example, one micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove unmasked portions thereof. The resulting three-dimensional silicon structure functions as a miniature mechanical force sensing device, such as an accelerometer that includes a proof mass suspended by a flexure. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. Nos. 5,006,487, “METHOD OF MAKING AN ELECTROSTATIC SILICON ACCELEROMETER”; and 4,945,765 “SILICON MICROMACHINED ACCELEROMETER”; and co-pending U.S. patent application Ser. No. 10/368,160, “MEMS ENHANCED CAPACITIVE PICK-OFF AND ELECTROSTATIC REBALANCE ELECTRODE PLACEMENT” filed in the names of Aiwu Yue and Ronald B. Leonardson on Feb. 18, 2003, the complete disclosures of all of which are incorporated herein by reference.
High aspect ratio MEMS or “HIMEMS” is one known process for producing such MEMS devices, including MEMS accelerometer devices. HIMEMS permits fabrication of intricate device designs in two dimensions, but requires a fixed device thickness, on the order of a cookie cutter.
Acceleration sensors fabricated using MEMS or HIMEMS technology generally include a moveable sensing element of the type illustrated in FIGS. 1, 4 and indicated respectively by the reference characters 7, 16. In FIGS. 2, 3 the moveable sensing element 7 is attached through attachment points 8 to the lower plate 2 which is a substrate on which the moveable sensing element 7 is manufactured. The lower plate or substrate 3 has formed thereon one set of the metal electrode layers 4, 5.
In FIGS. 4, 5 the moveable sensing element 16 is attached through attachment points 19 to a frame 20 which is a substrate in which the moveable sensing element 16 is manufactured. The lower substrate 18 over which the moveable sensing element 16 is suspended has formed thereon one set of the metal electrode layers 4, 5.
According to the current state of the art for fabricating conventional teeter-totter type acceleration sensor devices using MEMS or HIMEMS technology, each of the attachment points 8 for the one or more flexures 6 is formed as a “mesa” that is elevated relative to the bulk of the substrates 2 (or 3). Using MEMS or HIMEMS technology for fabricating conventional cantilevered type acceleration sensor devices of the type illustrated in FIG. 4 entails forming the one or more flexures 17 by which the acceleration sensing element 16 is suspended from the bulk of the substrate 20. A single etch step or operation thus constructs the respective flexures 6, 17 at attachment points 8, 19 and releases the silicon acceleration sensing elements 7, 16 from the bulk of the respective substrates 2 (or 3) and 20 for operation.
During the single etch step, the remainder of the substrate 2, 3, 18 is simultaneously formed with a substantially planar surface 9, 10 or 21, respectively, spaced by the distance D away from the respective acceleration sensing element 7, 16 when emplaced. The etching of the substrates 2, 3 and the frame 20 thus leaves respective attachment points 8, 19 spaced above the substantially planar substrate surfaces 9, 10 and 21. Thus, when emplaced on the elevated attachment points 8, 19, the respective acceleration sensing element 7, 16 is spaced a short distance away from the substrate surfaces 9, 10, 21 so that narrow gaps g1, g2 (best illustrated in FIG. 3), usually on the order of a few microns, for example on the order of 2–4 microns, wherein the acceleration sensing element 7, 16 is free to move during operation are formed between the substrate surface 9 (or 10) and 21 and surfaces of the acceleration sensing element 7, 16 on either side of the elevated attachment points 8, 19.
When intended for operation as a teeter-totter type accelerometer of the type illustrated in FIG. 1, a first portion 11 of the moveable sensing element 7 on one side of the fulcrum or hinge axis h of the flexure 6 is formed with relatively greater mass than a second portion 12 on the other side of the fulcrum or hinge axis h to develop a desired amount of pendulosity. The greater mass of the first portion 11 may be developed by offsetting the flexure 6 relative to the longitudinal dimension of the sensing element 7, as illustrated in FIG. 1, such that a tail portion 13 is provided distal from the flexure 6. In a device 1 fabricated using MEMS or HIMEMS technology, the sensing element 7 is necessarily a substantially two-dimensional object of substantially uniform thickness so that offsetting the flexure 6 causes the longer first portion 11 to have relatively greater mass than the shorter second portion 12 with a center of mass that is spaced relatively further from the fulcrum or hinge axis h of the flexure 6.
When intended for operation as a cantilevered type accelerometer of the type illustrated in FIG. 4, the entire mass of the moveable sensing element 16 suspended about the fulcrum or hinge axis h of the flexure 17 develops the desired pendulosity.
As is well-known in the art, the operating range of an accelerometer of the types illustrated in FIGS. 1 and 4 are physically limited to the acceleration or “g” force that overcomes the ability of the device to electrostatically balance the sensing element 7, 16 relative to the electrode layers 4, 5 and causes the respective teeter-totter and cantilevered type sensing elements 7, 16 to deflect relative to the surfaces 9, 10 and 21 of the respective substrates 2, 3 and 18. When this happens, the excitation and feedback electrodes 4, 5 detect the deflection, as a function of an imbalance in the sensed capacitance, and responsively drive the respective sensing element 7, 16 until it becomes rebalanced relative to the substrate surface 9, 10 and 21.
As discussed herein, obtaining high performance data from accelerometer output that has less than optimum linearity characteristics imposes significant obstacles on micromachined accelerometer designs. With respect to out of plane pendulous mass accelerometers that use stacked parallel plates for pick-off, obtaining linear output has been difficult because the capacitance varies inversely with displacement (1/d). For a teeter-totter type acceleration sensing device 1 of the type illustrated in FIGS. 1, 2, 3, state of the art design and fabrication techniques assume that the fulcrum or hinge axis h of the pendulous acceleration sensing element 7 is coincident with the geometric centerline Cg of the flexures 6. Therefore, state of the art design and fabrication techniques include centering excitation and feedback electrodes 4, 5 about the geometric centerline Cg of the flexures 6 which is one half the flexure length FL, written as FL/2, when the flexure is of a rotational, i.e., teeter-totter, configuration. Thus, according to the state of the art in fabricating conventional mid-pendulum hinged or “teeter-totter” type accelerometers of the type illustrated in FIG. 1, the fulcrum or hinge axis h is assumed to be located at the geometric centerline Cg of the flexure 6, and the effective portions of excitation and feedback electrodes 4, 5 are positioned relative to the geometric centerline Cg of the flexure 6, which is half way between the first and second portions 11, 12 of the moveable sensing element 7 on opposite sides of the assumed fulcrum or hinge axis h.
In other words, the effective portions of first and second electrodes 4, that part of the electrode that is covered by a portion of the sensing element 7, are equidistant from the geometric centerline Cg of the flexure 6 such that the respective center points CL4 of the effective portions of the two electrodes 4 are each spaced an equal distance d4 from the geometric centerline Cg of the flexure 6. Similarly, the effective portions of first and second electrodes 5 are also equidistant from the assumed fulcrum or hinge axis h such that the respective center points CL5 of the effective portions of the two electrodes 5 are each spaced an equal distance d5 from the geometric centerline Cg of the flexure 6. If a single set of electrode elements 4 (or 5) operates as both excitation and feedback electrodes, the electrode elements 4 (or 5) are spaced an equal distance d4 (or d5) from the flexure geometric centerline Cg.
Furthermore, state of the art design and fabrication of conventional mid-pendulum hinged or “teeter-totter” type accelerometer devices 1 assumes that the geometric centerline Cg of the flexure 6 continues to operate as the fulcrum or flexure hinge axis h over the entire operating range of the device 1 such that the fulcrum or hinge axis h is assumed to remain at the flexure geometric centerline Cg throughout the range of deflection of the sensing element 7 until it becomes rebalanced relative to the substrate surface 9, 10. In other words, conventional design and fabrication techniques for mid-pendulum hinged type accelerometers assume that the flexure length FL, as measured parallel to a neutral axis n of the relaxed and undeflected flexure 6, remains constant over the entire dynamic operating range of the device 1 so that the position of the geometric centerline Cg of the flexure 6 relative to the electrode elements 4, 5 remains the same when deflected.
According to the state of the art in fabricating conventional bending beam or cantilever-hinged pendulum type accelerometers, of the type illustrated in FIG. 4, the effective portions of excitation and feedback electrodes 4, 5 are positioned relative to the geometric centerline Cg of the flexure 17, which is half way between the flexure attachment point 19 to the frame 20 and a distal end 22 of the flexure 17 where it attaches to the pendulous acceleration sensing element 16. In other words, the effective portions of the electrodes 4, 5 are positioned as a function of the geometric centerline Cg of the flexure 17 such that the effective portions of electrodes 4, 5 are spaced respective distances d4, d5 from the geometric centerline Cg of the flexure 17.
Furthermore, state of the art techniques for design and fabrication for bending beam or cantilever-hinged pendulum type accelerometers also assume that the position of the geometric centerline Cg flexure 17 remains constant relative to the electrode elements 4, 5 throughout the range of deflection of the sensing element 16 until it becomes rebalanced relative to the substrate surface 21. In other words, the flexure geometric centerline Cg of the relaxed or undeflected flexure 17 relative to the electrode elements 4, 5 is assumed to remain the same when the sensing element 16 is deflected.